As more functionalities are implemented in a small mobile devices such as cell-phones and notebook-size personal computers, a secondary battery used as a power source thereof is expected to have a larger energy density. As a secondary battery that has a larger energy density, a non-aqueous solvent-type lithium secondary battery which utilizes a carbonaceous material as an anode has been proposed (Patent Document 1).
In recent years, large secondary batteries, having large energy density and excellent output energy characteristics are mounted on electric vehicles in response to increasing concern over environmental issues. For example, use of such secondary batteries is anticipated in vehicle applications such as in electric vehicles (EV), which are driven solely by motors, and plug-in hybrid electric vehicles (PHEV) or hybrid electric vehicles (HEV) in which internal combustion engines and motors are combined. In particular, a lithium-ion secondary battery, which is a non-aqueous solvent-type lithium secondary battery, is used widely as a secondary battery having a large energy density. Further increase in energy density is expected so that a longer cruising distance at one charging can be achieved for the EV use.
Although larger energy density requires an increase in doping and de-doping capacity of lithium into an anode material, the theoretical lithium impregnating capacity for a graphitic material, which is commonly used as an anode material today, is 372 Ah/kg. This is the theoretical upper limit for a graphitic material. Furthermore, if a graphitic material is used to configure an electrode, a graphite intercalation compound is formed upon lithium doping into the graphitic material, thereby increasing a layer spacing. De-doping of the lithium doped between layers can restore the layer spacing. Accordingly, such an increase and restoration in the layer spacing may occur repeatedly in a graphitic material with a developed graphite structure, by repeated events of doping and de-doping of lithium (i.e. repeated charging and discharging in the case of a secondary battery), leading to a possible disintegration of graphite crystal. Therefore, it is speculated that the secondary battery configured with a graphite or a graphitic material with a developed graphite structure exhibits inferior performance in repeated charging and discharging. For a battery in which a graphitic material with a developed graphite structure is used, another problem is also addressed: an electrolyte solution tends to decompose during the battery operation.
Alternatively, an anode material of alloy-type, such as tin and silicon, which exhibits large capacity, is also proposed. However, it does not have enough durability, and its usage is limited.
In contrast, a non-graphitic carbon material exhibits superior durability and possesses a greater capacity per weight than the theoretical lithium impregnating capacity of a graphitic material. Therefore, various proposals as a large-capacity anode material have been made so far. The use of a carbonaceous material obtained by subjecting phenol resin to heat treatment as an anode in a secondary battery has been proposed, for example (Patent Document 2). However, there has been a problem with such a carbonaceous material. An anode produced using a carbonaceous material obtained by subjecting phenol resin to heat treatment at a high temperature (e.g. 1900° C. or higher) can achieve only small doping and de-doping capacity of an active material such as lithium into an anode material. On the other hand, if an anode is produced using a carbonaceous material obtained by subjecting phenol resin to heat treatment at a relatively low temperature (e.g. from approximately 480° C. to 700° C.), the doping capacity of lithium, which is an active material, is large. This material is preferable in this aspect. However, lithium doped into an anode carbon may not be completely de-doped and a large amount of lithium may remain in the anode carbon. Such a wasteful consumption of lithium, which is an active material, is a problem.
Alternatively, a method of manufacturing a carbon for a lithium secondary battery is proposed (Patent Document 3). The method includes:
contacting a dry-distilled charcoal with a gas which contains halogen thereby providing a halogenated dry-distilled charcoal;
removing a portion or all of said halogen in said halogenated dry-distilled charcoal thereby obtaining de-halogenated charcoal; and
contacting a thermally decomposable hydrocarbon with said de-halogenated charcoal, thereby adjusting the pores of the carbon product prepared. With this method, a large doping and de-doping capacity can be achieved. However, lithium doped into an anode carbon may not be completely de-doped and a large amount of lithium may remain in the anode carbon. Such a wasteful consumption of lithium, which is an active material, is a problem.